One of the most disturbing aspect of teaching people is having to watch people jump in and out of sarvangasana without any style. Unfortunately this lack of structure is more common amongst Indian students.Perhaps too much use of props, or maybe lack of overall fitness, or simply an inability to connect with the body in a manner that comes to those nationalities or people's who are sporty by nature.
Given this there are also contraindications for those who hold inversions for too long. This too must be a local problem..a general sense of not seeing a sequence in their asana practice and lumping practices any which way.
Here is a warning by Swami Satyanandaji who I consider my inner guru in these matters.
Ideal duration for normal people is 3 to 5 minutes.
If you have impure blood u must avoid long stay in such poses. One way to know this is if u have boils or are constipated.
Also to avoid if u feel dizzy. Or are perspiring unnaturally, have unnatural heart palpitations.
Though inversions have their position in any sequence sarvangasana is in a class of its own. Because it can induce tranquility and yet keep the mind alert it is advised both in the beginning and end of asana practice and just before pranayama.